U.S. Pat. No. 4,429,990, issued Feb. 7, 1984 to E. J. Tamary, shows an electrophotographic copier which forms a series of toner images, transfers those images to the opposite sides of receiving sheets and fuses both toner images to the receiving sheets simultaneously. The fuser in the Tamary apparatus is a heated roller fuser with both rollers heated. In a commercially successful application of this fuser, the rollers each have elastomeric coverings. To control the temperature of the rollers, a metallic core is sensed outside the image area of each roller and a heating lamp is regulated in each roller according to the temperature sensed.
In simplex operation, the apparatus feeds sheets to the fuser at a constant full machine rate with a small interframe between the sheets. In this condition, a large amount of heat is absorbed by the paper from the fuser. Accordingly, "run" set points during a continuous simplex run are relatively high. During duplex operation, the sheets are fed to the fuser also at a constant rate but one-half that of the simplex rate. There thus is a space of at least one sheet between sheets. As a result, "run" set points for duplex are somewhat lower than for simplex.
When the apparatus is on but no copies are being made, the temperature set points are at a lower "standby" temperature. Each temperature set point is designed to produce a temperature of 340.degree. F. at the fusing surface of the rollers when they are contacting images. The higher set points during run in either simplex or duplex are designed to maintain that temperature despite heat being carried out of the fuser by the sheets.
This commercial copier produces excellent copies with this approach. However, in applying this principle to a similarly designed highspeed, high-quantity printer, it was found that occasionally the speed of the printer exceeded the ability of the printer electronics to format image pages. Accordingly, in the middle of a run, the printer would stop printing for one or more frames. If the fuser was set at its run set points, but no sheets were coming through the fuser, the surface of the fusing rollers have a tendency to overheat. Using toners with a fusing temperature of 340.degree. F., serious problems were not caused. However, in adapting the printer to a higher fusing temperature toner, for example, 380.degree. F., such overheating occasionally would activate a temperature shutdown sensor, char the paper or cause hot offset of the toner.
Frames containing no image (sometimes called "skip frames") commonly occur when a raster image processor that converts information from, typically, ASCII Code, into a bit map does not keep up with the process speed of the printer. Similar situation at a fuser will occur if multicolor images are being formed in which a number of images are combined on a single side of a sheet, thereby reducing the throughput of the fuser during a run.